Fluid injectors are in widespread use, in particular for internal combustion engines where they may be arranged in order to dose fluid into an intake manifold of the internal combustion engine or directly into the combustion chamber of a cylinder of the internal combustion engine.
In order to enhance the combustion process in view of the creation of unwanted emissions, the respective fluid injector may be suited to dose fluids under very high pressures. The pressures may be in case of a gasoline engine, for example, in the range of up to 200 bar and in the case of diesel engines in the range of up to 2000 bar.
WO 2011/000663 A1 discloses a fluid injector comprising a longitudinal axis and a valve needle, which is axially moveable and operable to prevent a fluid injection in a closing position and to permit the fluid injection in further positions. The fluid injector also comprises an armature being mechanically coupled to the valve needle, and a solenoid assembly which comprises at least a first and second coil and which is operable to magnetically actuate the armature via an electrical signal applied to at least one predetermined assortment of the at least two coils. This enables an adjustment of the fluid injection to the current operating conditions, in particular a fluid pressure, of the fluid injector. Applying the electrical signal on a first predetermined assortment comprising more than one coil contributes to increasing the solenoid inductance and the magnetic force acting on the armature. This permits the fluid injection in a fast manner. On the other hand, if the fluid pressure within the fluid injector is relatively low the electrical signal may be applied to a second predetermined assortment comprising less coils than the first assortment. This reduces e.g. ohmic drops due to reduced resistance and contributes to ensuring an efficient operation of the fluid injector.
Due to always more stringent requirements, the solenoid injector must be controllable in order to deliver very small fuel quantities. In particular, this is true for solenoid injectors under so called ballistic operating mode. To control the injector, an electrical feedback signal is used to detect the movement changes of an injector armature when the armature-needle assembly reaches a fully opened and a fully closed position. Evaluating this signal with an appropriated controlling unit makes it possible to control minimum dispensable fuel delivery quantities. The electrical feedback signal is measured between the terminals of a coil which is used to generate a magnetization of the armature in order to open and close an injector valve.
In order to achieve a good signal quality of the electrical feedback signal available from the injector circuit, the injector body needs to have a restriction (a thin valve body section) in the area of the coil which supports the electrical signal development to detect the closing position of the armature-needle assembly. This design requires that the valve body is made by special “not good” magnetic steel, for example 415M SS, with limited saturation level at around 1 Tesla. As a disadvantage, this has the effect that the electrical signal amplitude will be reduced. Nevertheless, a valve body having a section with reduced thickness must accomplish all requirements coming with regard to the structural resistance. Hence, the material of the valve body must also support higher mechanical stresses.